Radionuclide imaging devices, such as gamma cameras, are used in the medical field to measure radioactive emissions emanating from a subject's body and to form a comprehensible output from these measurements, typically in the form of an image that graphically illustrates the distribution of the emissions within the patient's body. The emissions originate from a decaying radioactive tracer that has been intentionally introduced into the subject's body, and therefore, the image produced by the radionuclide imaging device represents the distribution of the tracer within the subject's body. The radioactive tracer is a pharmaceutical compound to which an electromagnetic radiation emitting nuclide, such as 99mTc, has been attached and which undergoes a physiological process after introduction into the body and exhibits an affinity for a certain organ or tissue.
The radionuclide imaging device has one or more detectors that detect the number of emissions, generally gamma rays in the range of 140 keV. Each of the detected emissions is a “count,” and the detector determines the number of counts at different spatial positions. The imager then uses the count tallies to form an estimate of the distribution of the tracer, typically in the form of a graphical image having different colors or shadings that represent the count tallies.
Radionuclide imaging devices have traditionally used homogeneous area radiation detectors. For example, U.S. Pat. No. 3,011,057 for RADIATION IMAGE DEVICE, hereby incorporated by reference in its entirety, describes a radiation imager that uses a single NaI scintillation detector crystal to detect gamma ray emissions. The NaI radiation detector is generally characterized by spatial resolution and energy resolution performance that is substantially uniform across the relatively large area of the NaI detector crystal surface.
FIG. 1 provides a schematic diagram of a traditional NaI radiation detector 10 that generally comprises a detector 4, such as a scintillation crystal, for transforming gamma ray emissions to light photons in response to incident gamma ray events, and a photodetector 6 to detect the light photons emitted from the scintillator. The photodetector 6, typically a photomultiplier tube that is optically coupled to the scintillator 4, detects a fraction of the scintillation photons produced from absorption of a gamma ray into the scintillation crystal and produces an electronic current that is proportional to the number of detected scintillation photons.
In one known technique used with NaI detectors, the radionuclide imager forms a high-resolution image through the use of a small-aperture collimator that provides collimated gamma ray paths to the detector. In this technique, the position of the gamma ray at the point of absorption in the scintillation crystal is determined by an algorithm based on the magnitude of electric signals from each of a plurality of photomultiplier tubes 6 positioned over the crystal. This algorithm can be implemented by use of a resistor matrix connecting the outputs of the photomultiplier tubes. For close proximity images, a single long-bore, small-aperture collimator hole can be used, with the collimator being scanned over the radiation field of interest in a two-dimensional scanning manner, to thereby sample radiation distribution over each of the image points in the radiation field. Multiple holes can be used to increase the number of counts obtained at each point, provided they are sufficiently separated from each other such that detected counts can be associated with a particular collimator hole.
For the radionuclide imager to form a high-resolution image, the detector must be able to distinguish between the photons received through each of the collimator holes. If the collimator holes are spaced too closely together, the spatial resolution advantage gained by using multiple collimator holes is lost because the intrinsic uniform spatial resolution and energy resolution of the NaI detector blurs the discernable location of a detected radiation emission such that detected radiation emission could have passed through any of several collimator holes. As a result, the density of the packing of the collimator holes is limited by distance of separation needed by the homogeneous NaI detector to achieve desired spatial resolution. This concept of separating multiple collimator holes by a sufficient distance to produce uniquely identifiable locations is known in the art and is described, inter alia, in U.S. Pat. Nos. 3,752,982 and 3,784,821, both issued to Jaszczak, and in International Application No. WO 00/38197 filed by Boxen, incorporated herein by reference. Because the collimator holes are sufficiently separated to allow the detector to identify the hole of origin for each detected radiation event, this imaging concept is henceforth referred to as the “sparse hole” technique.
The sparse hole imaging technique generally requires a precise motion of the collimator within a two-dimensional plane in order to obtain the usable counts needed to form images. This requirement for precise motion of the collimator throughout the sampling area adds considerable complexity to the design of the radionuclide imager. As a result, there exists a present need for a radionuclide imager having a relatively simple design that can move the collimator holes throughout a desired sampling area to obtain radiation counts while preserving the correct spacing necessary to produce high resolution images.
In addition to the above-described NaI detectors, radionuclide imagers with pixellated radiation detector elements, typically cadmium zinc telluride (“CZT”) crystals, have recently been developed. In these pixellated radionuclide imagers, the intrinsic spatial resolution is defined by the size of the individual pixellated detector elements, rather than the separation between collimator holes. However, the pixel elements are nonhomogeneous in response and tend to have the best performance in the center of the pixel, with poorer performance at the boundaries between pixels. It can therefore be advantageous to have collimator holes allowing photons to interact with the centers of the pixels, where the collimator body masks the pixel-pixel boundaries. This idea of aligning the collimator holes with the centers of the pixels is henceforth referred to as the “registered collimator” concept and is analogous in operation to the above described sparse hole technique.
A typical pixellated radiation detector 20 is schematically illustrated in FIG. 2. The pixellated detector 20 is generally characterized by multiple detector elements 12. Each of the detector elements 12, as described above, has a center region 13 of higher detector performance. In order to direct radiation to the center regions 13 of the detector elements 12, the pixellated detector 20 further comprises a collimator 18 containing collimator holes 19 that correspond to the position of the individual detector elements 12.
Like the sparse hole imaging technique, registered collimator scanning generally requires a precise motion of the collimator holes within a two-dimensional plane in order to obtain the usable counts needed to form images, adding considerable complexity to the design of a radionuclide imager using pixellated detector elements. As a result, there exists a further need for a radionuclide imaging technique that can be adapted for use with registered collimator pixellated detector elements without adding complexity to the design and operation of the imager.
As described above, it is known in the field of radionuclide imaging devices to form imaging detectors by precisely positioning the collimated radiation detectors to scan a two-dimensional area by using either a sparse hole NaI detector or a pixellated CZT detector. In an imager having these types of multiple aperture, collimated detectors, the image resolution is ultimately limited by the definition of the collimator aperture in the individual detector elements. In particular, a detector element having a long bore, small diameter collimator aperture can produce superior spatial resolution, at the cost of reducing the number of gamma rays that are capable of traversing the aperture to be counted by the detector, thereby decreasing the sensitivity of the detector.
With radionuclide imagers having highly collimated detector elements, the spatial resolution performance rapidly degrades as distance increases between the detector's surface and the source of radiation. One cause for this degradation of spatial resolution is that the radiation emissions are not parallel and, as a result, become increasingly commingled as they move farther from the radiation source. Therefore, the radionuclide imager forms the best resolution images when the radiation sources are positioned in close proximity to the collimated detector elements. Accordingly, there exists a further need for a radionuclide imager that positions the collimated detectors in close proximity to the target to be scanned.
It is further known in the field of radionuclide imaging that the performance of the imager can be improved through the use of multiple radiation detectors. The use of multiple detectors is advantageous because the radionuclide imager may collect samples from a target in less time. An imager having two detectors, for instance, may scan a target twice as fast as an imager having a single detector. Furthermore, the use of multiple detectors to scan a target may improve the resolution of the scanning by reducing the variance and resulting statistical error produced by a single detector. However, configuring the multiple detectors for precisely scanning throughout a sampling area, such as required for sparse hole or registered collimator imaging, adds still greater complexity to the design for the radionuclide imager. As a result, there exists a further need for a radionuclide imager having multiple detectors and a relatively simple design.
Also, it has become increasingly important to perform high-resolution scanning of three-dimensional objects. For example, the accurate radionuclide imaging of small animals allows improved veterinarial diagnosis and superior results in scientific research. Likewise, the accurate radionuclide imaging of a body part, such as a breast, may aid doctors in providing more accurate medical diagnosis. While the NaI and CZT collimated radiation detectors are relatively simple, inexpensive devices that provide accurate, high-resolution count information from a planar sample area, these types of detector devices have not been adapted for use in scanning of three-dimensional targets. There accordingly exists a further need for a radionuclide imager that can scan and image three-dimensional objects using known high-resolution collimated radionuclide detectors.